This invention relates to soldering of electronic components and, in particular, to a method and apparatus for removal of contaminants from the solder during the solder operation.
In a typical printed circuit board fabrication process, the boards are coated with a solder mask which leaves exposed the copper contact pads and plated through-holes to be soldered. The boards are then dipped vertically in, or transferred horizontally through, molten solder and, upon removal from the solder pot, the excess solder is discharged by directing jets of air at the board in a procedure generally referred to in the art as Hot Air Solder Leveling (HASL). (See, e.g., U.S. Pat. No. 5,110,036 issued to Parker.)
As circuit boards are dipped into the solder pot, the solder, which is typically lead-tin, becomes contaminated with dissolved copper from the pads on the boards. When the contamination reaches a certain level (typically 0.3 percent), some solder is removed and replaced by fresh solder to lower the contamination level (usually to about 0.2 percent). The removed solder is usually returned to the solder supplier for recycling.
This approach is fairly expensive since the removed solder is classified as a hazardous waste and the recycle fee is high (currently $1.02 per pound). In addition, process downtime is required to remove the contaminated solder and melt the fresh solder.
One recent proposal involves a continuous removal of copper from solder. (See Japanese Patent 1-168826.) An auxiliary pot is provided with a pump and is connected to the main solder pot by means of pipes. The solder is drained out of the main pot and, while flowing in the auxiliary pot, is cooled in order to precipitate the copper. The purified solder is then pumped back into the main pot.
While generally useful, this approach has certain disadvantages. For example, if the solder level drops in the main pot as often occurs in the HASL process whenever the main solder pump is started and at other times, the solder could stop flowing in the auxiliary pot. Also, the maximum depth of solder in the auxiliary pot is limited to that of the main pot thereby increasing the space required for the auxiliary pot. Further, this scheme requires a pump in the auxiliary pot in addition to the standard pump in the main pot.